The present invention relates to access to a digital document in a communication network, in particular of the station to station type, the said document comprising at least first and second data connected to each other in a chosen hierarchical relationship.
It finds a general application in rapid reliable access to digital data distributed in a communication network and more particularly in a communication network of the station to station type.
Public communication networks are known, such as the Internet, which are widely used as means of exchanging information throughout the world.
In the so-called “conventional” distribution mode, a client device connects to a distant server in order to obtain information. This information is named so that the distant server device is clearly identified. For example, an address or location, also referred to as URL according to the English acronym “Uniform Resource Locator”, is associated with a resource (a data file) in order to identify the precise address of the Internet server device storing this resource.
In a distribution mode of this type, the latency caused with regard to the access to the data results directly in the time necessary for recovering the data from the distant server device to the client device.
So-called “station to station” or distributed communication networks are also known, commonly referred to as “peer to peer” topology in English.
In a station to station network, the equipment puts resources in common according to an equal to equal sharing philosophy. Thus each machine in a station to station network behaves both as a client and as a server.
Such networks are thus distinguished from the conventional “client-server” networks in which the resources are archived in a central server device.
At the present time the distribution of data is more and more having recourse to station to station networks because of their low establishment cost, the presence of very many computers connected to the network, and the development of high-rate connections.
However, station to station networks are unstable. This is because client devices (and consequently server devices) connect or disconnect periodically to the network, thus making the presence of the data very haphazard. In addition, the addresses of the client and/or server devices are unpredictable and liable to be different at each connection.
As a result, access to the contents in a communication network of the station to station type still constitutes a significant difficulty since the latency in obtaining the data is no longer simply due to the time needed for recovering the data as in conventional client-server topology but also the time for searching for a server device having this data available. According to the topology of the station to station network concerned, this search phase may be not insignificant.
A known solution for improving access to data in terms of speed consists of using a server device known as a “proxy-cache server” or proxy server with cache which serves as an intermediary between the computers in a local network and the Internet.
In practice it is a case of a server “authorized” by an application to make a request on the Internet in its place. Thus, when a user connects to the Internet by means of a client application configured for using a proxy server device, the said application will firstly connect to the proxy server device and give it its request. The proxy server device will then connect to the server device which the client application is seeking to join and will transmit the request to it. The server device will then give its response to the proxy server device, which will in its turn transmit it to the client application.
The majority of proxy server devices thus fulfill a cache function, that is to say the ability to keep in memory (in “cache”) the pages most often visited by the users of the local network in order to be able to supply them to them as quickly as possible.
Thus a proxy server device with cache on the one hand reduces the use of the bandwidth to the Internet and reduces the access time to the documents for the users.
However, in order to succeed in this task, it is necessary for the proxy server device to regularly compare the data which it is storing in cache memory with the distant data in order to ensure that the data in cache are still valid.
As a result a proxy-cache server device is advantageous in repetitive access to the same data not only for the speed of access but also for reducing the Internet traffic.
However, a proxy-cache server device is ineffective for first access to data.
The solution known as searching by anticipation, or “prefetching” in English, is also known, which is based on the idea that, whilst a user is browsing on the Internet, the pages which he is assumed to display in the very near future are downloaded automatically in anticipation of their being requested by the user.
Thus, at the time of the actual request, the program browsing on the Internet can immediately display the data downloaded in anticipation. This method limits the data access latency time.
However, this solution is not completely satisfactory since very often the data downloaded in anticipation are never used, which means that the method unnecessarily overloads the consumption of the network bandwidth.
As a result, in systems using such a solution, this increase in load contributes to an increase in the waiting time for the requests of the user (requests by the user for non-anticipated data), which has the opposite effect to that sought and degrades performance. In addition, the undesirable effect on the Internet (the addition of network traffic and overloading of the servers) would be not insignificant if all the browsers on the Internet used such a method of search by anticipation.